Essentially, animals are many-celled heterotrophic organisms. Plants and algae characteristically manufacture their food from inorganic substances (usually by photosynthesis); animals must secure food already organized into organic substances. They are dependent upon photosynthetic organisms, which provide oxygen as a byproduct and are the ultimate source of all their food. Animals (as well as plants) provide carbon dioxide through respiration and the decomposition of their dead bodies (see carbon cycle; nitrogen cycle). In addition, most animals have specialized means of locomotion, generally involving muscle cells, and possess nervous systems and sense organs—all adaptations for securing food. In most forms there is a distinct alimentary canal or digestive system. Animal cells do not have cell walls. Almost all animals, unlike most plants, possess a limited scheme of growth; that is, the adults of a given species are nearly identical in their characteristic form and are similar in maximum size. Most animals reproduce sexually, but some are capable of asexual reproduction under certain circumstances.
With the advent of electron microscopy and advanced biochemical analyses, intricate differences between simple and microscopic organisms were better understood, and many that were previously fit into the animal or plant kingdom were then placed into separate kingdoms (i.e., Monera for the bacteria, Protista for the algae and protozoans, and so forth). In zoological classification the animal kingdom has been divided into the three subkingdoms of Parazoa (the sponges), Mezozoa (wormlike parasites), and Eumetazoa. Eumetazoa comprises numerous invertebrate phyla and the phylum Chordata. The chordates include two primitive subphyla of a few species each and the subphylum Vertebrata (see vertebrate). There are at least 1.5 million animal species; approximately 95% of these are invertebrates.
The scientific study of animals is called zoology; the study of their relation to their environment and of their distribution is animal ecology. For specific approaches to the study of living things, see biology.
Aquatic invertebrate of the phylum Bryozoa (“moss animals”), members (called zooids) of which form colonies. Each zooid is a complete and fully organized animal. Species range in size from a one-zooid “colony” small enough (less than 0.04 in., or 1 mm, long) to live between sand particles to colonies that hang in clumps or chains as much as a 1.6 ft (0.5 m) across. The texture of colonies varies from soft and gelatinous to hard with calcium-containing skeletons. Freshwater bryozoans attach primarily to leaves, stems, and tree roots in shallow water. Marine bryozoans have a wide range of habitats, from coastal areas to great ocean depths, but are most common just below the tidemarks. Bryozoans feed by capturing plankton with their tentacles.
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Farm animals, with the exception of poultry. In Western countries the category encompasses primarily cattle, sheep, pigs, goats, horses, donkeys, and mules; other animals (e.g., buffalo, oxen, or camels) may predominate in other areas. Seealso ass, cow, dairy farming.
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Act of offering objects to a divinity, thereby making them holy. The motivation for sacrifice is to perpetuate, intensify, or reestablish a connection between the human and the divine. It is often intended to gain the favour of the god or to placate divine wrath. The term has come to be applied specifically to blood sacrifice, which entails the death or destruction of the thing sacrificed (see human sacrifice). The sacrifice of fruits, flowers, or crops (bloodless sacrifice) is more often referred to as an offering.
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Offering of the life of a human being to a god. In some ancient cultures, the killing of a human being, or the substitution of an animal for a person, was an attempt to commune with the god and to participate in the divine life. It also sometimes served as an attempt to placate the god and expiate the sins of the people. It was especially common among agricultural people (e.g., in the ancient Near East), who sought to guarantee the fertility of the soil. The Aztecs sacrificed thousands of victims (often slaves or prisoners of war) annually to the sun, and the Incas made human sacrifices on the accession of a ruler. In ancient Egypt and elsewhere in Africa, human sacrifice was connected with ancestor worship, and slaves and servants were killed or buried alive along with dead kings in order to provide service in the afterlife. A similar tradition existed in China. The Celts and Germanic peoples are among the European peoples who practiced human sacrifice.
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rights, primarily against being killed and being treated cruelly, that are thought to be possessed by higher nonhuman animals (e.g., chimpanzees) and many lower ones by virtue of their sentience. Respect for the welfare of animals is a precept of some ancient Eastern religions, including Jainism, which enjoins ahimsa (“noninjury”) toward all living things, and Buddhism, which forbids the needless killing of animals, especially (in India) of cows. In the West, traditional Judaism and Christianity taught that animals were created by God for human use, including as food, and many Christian thinkers argued that humans had no moral duties of any kind to animals, even the duty not to treat them cruelly, because they lacked rationality or because they were not, like Man, made in the image of God. This view prevailed until the late 18th century, when ethical philosophers such as Jeremy Bentham applied the principles of utilitarianism to infer a moral duty not to inflict needless suffering on animals. In the latter half of the 20th century, the ethical philosopher Peter Singer and others attempted to show that a duty not to harm animals follows straightforwardly from simple and widely accepted moral principles, such as “It is wrong to cause unnecessary suffering.” They also argued that there is no “morally relevant difference” between humans and animals that would justify raising animals, but not humans, for food on “factory farms” or using them in scientific experiments or for product testing (e.g., of cosmetics). An opposing view held that humans have no moral duties to animals because animals are incapable of entering into a hypothetical “moral contract” to respect the interests of other rational beings. The modern animal-rights movement was inspired in part by Singer's work. At the end of the 20th century, it had spawned a large number of groups dedicated to a variety of related causes, including protecting endangered species, protesting against painful or brutal methods of trapping and killing animals (e.g., for furs), preventing the use of animals in laboratory research, and promoting what adherents considered the health benefits and moral virtues of vegetarianism.
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Foodstuff grown or developed for livestock and poultry to maintain the health of the animals and to increase the quality of such end products as meat, milk, or eggs. Modern feeds are derived from crops grown specifically for research or from by-products of surplus crops or foods produced for human consumption. Feeds are categorized as either concentrates (high in digestibility of nutrients but low in fibre content) or roughages (high in fibre and comparatively low in digestive nutrients). Most diets consist of a combination of feeds.
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Transmission of information from one animal to another by means of sound, visible sign or behaviour, taste or odour, electrical impulse, touch, or a combination of these. Most animal communication uses sound (e.g., birds calling, crickets chirping). Visual communication usually indicates an animal's identity (species, sex, age, etc.) or other information through specific characteristics (e.g., horns, patches of colour) or behaviour (e.g., the bee's “dance” describing a source of food). Chemical communication involves pheromones (chemical signals) produced by the animal's endocrine system. Eels and some other fishes use electrical impulses to communicate.
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Any member of the kingdom Animalia (see taxonomy), a group of many-celled organisms that differ from members of the two other many-celled kingdoms, the plants and the fungi (see fungus), in several ways. Animals have developed muscles, making them capable of spontaneous movement (see locomotion), more elaborate sensory and nervous systems, and greater levels of general complexity. Unlike plants, animals cannot manufacture their own food, and thus are adapted for securing and digesting food. In animals, the cell wall is either absent or composed of material different from that of the plant cell wall. Animals account for about three-quarters of living species. Some one-celled organisms display both plant and animal characteristics. Seealso algae, arthropod, bacteria, chordate, invertebrate, protist, protozoan, vertebrate.
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